Fuel cells convert the chemical energy of fuels directly into electricity. The principle of the fuel cell was developed by William Grove in 1839. The alkaline fuel cell is one of the oldest and most simple type of fuel cell. This is the type of fuel cell that has been used in space missions since it first being developed at 1940. Since then, a variety of fuel cells had been development for all kinds of commercial usages. Among those, solid oxide fuel cells (SOFCs) are particularly attractive because they have the highest efficiencies of any conventional fuel cell design and the potential to use many fuels, including methane, without expensive external reformers that create more volatile chemicals. SOFCs can operate at high temperatures, producing high-grade waste heat, or exhaust, which can be recovered and used for other applications, such as space heating and cooling, supplying homes with hot water, and even generating extra electricity by spinning a gas turbine linked to the unit.
Conventionally, an SOFC is constructed with two porous electrodes which sandwich an electrolyte. In an SOFC, fuel, e.g. methane, and oxidant, e.g. air, are preheated to a temperature close to the operating temperature of the SOFC, i.e. between 600° C.˜1000° C., and then being fed into the SOFC. When an oxygen molecule contacts the cathode/electrolyte interface as the air flows along the cathode (which is therefore also called the “air electrode”), it catalytically acquires four electrons from the cathode and splits into two oxygen ions. The oxygen ions diffuse into the electrolyte material and migrate to the other side of the cell where they encounter the anode (also called the “fuel electrode”). The oxygen ions encounter the fuel at the anode/electrolyte interface and react catalytically, giving off water, carbon dioxide, heat, and—most importantly—electrons. The electrons transport through the anode to the external circuit and back to the cathode, providing a source of useful electrical energy in an external circuit. Furthermore, the exhaust air with temperature higher than 700° C. and residual fuel, both being discharged at the exit of the SOFC, can be recycled for other usages.
Two possible design configurations for SOFCs have emerged: a planar design and a tubular design. Since the planar SOFCs are troubled by the difficulty of keeping airtight, tubular SOFC had been developed starting from 1960 by Westinghouse Electric Corp as disclosed in U.S. Pat. Nos. 4,490,444, 4,833,042, 6,416,897 and 6,444,342. In the tabular SOFC, components, i.e. the fuel electrode, the electrolyte and the air electrode, are assembled in the form of a hollow tube so that the tabular SOFC can keep good airtight even when subjecting to a high-temperature ambient even at 1000° C., but is suffered by the problems of high fabrication cost due to the complicated process required to manufacture the same and high internal impedance due to the path of current generated thereby is comparatively longer. Since the voltage output of a single tubular fuel cell is far to low for many applications, it frequently becomes necessary to connect multiple tubular fuel cells in series, parallel or series/parallel configuration while arranging the plural tubular fuel cells neighboring to each other as those disclosed in US. Pat. No. 4,490,444. However, as the increase of output voltage, the overall volume of the assembly of the plural tabular cells is increase that might not be a good idea in the usage point of view. It is noted that tubular designs have a drawback of low volumetric power packing density. In other words, in order to generate an equivalent amount of power, a tubular fuel cell is generally much larger in size than a planar fuel cell.
Although, a space-saving tubular SOFC with assembly of concentrically arranged electrodes had already been disclosed in U.S. patent application No. 2004/0258972, which is advantageous in that the size of the space-saving tubular SOFC of U.S. patent application No. 2004/0258972 is significantly smaller than other stacked designs, it is still lack of functions of fuel recycling, gas preheating and modularization.
Therefore, it is in great need to have a compact solid oxide fuel cell module that is free from the problems of conventional fuel cells, but still keeps all the benefits that conventional fuel cells have.